The proposed research is a combined genetic and biochemical study of mitochondrial biogenesis in Neurospora with the long range goals of defining the roles of the nuclear and mitochondrial genetic systems and understanding how these roles are coordinated and modulated. Specific aims focus on mitochondrial ribosome assembly. We propose to isolate mutants with defects in mitochondrial ribosome assembly and then, relying in part on techniques developed in previous studies, to analyze the most interesting mutants biochemically. We will attempt to identify specific lesions in mitochondrial rRNAs and proteins and to correlate these with defects in mitochondrial ribosome assembly and protein synthesis. In addition, we will attempt to identify mutants with defects in regulatory proteins which coordinate the two genetic systems during the assembly process. A number of specific goals relate to poky (mi-1), and extra-nuclear mutant with an assembly defect leading to a deficiency of mitochondrial small ribosomal subunits. In previous work, we identified one mitochondrial ribosomal protein (S-4a, apparent MW 52,000) which is synthesized intra-mitochondrially and which may be the site of the primary defect in poky. Major objectives of the proposed research are to continue efforts to identify the primary defect in poky and to determine the role of S-4a in mitochondrial ribosome assembly and protein synthesis.